As the informatization of the society advances, information leakage has become a serious issue, and there has been a need for development of techniques for preventing information leakage. For example, for digital data distributed on the Web, Internet and electronic storage media, techniques for encrypting data has been developed so that even if information is caught by a third party, its contents are not to be seen, and has already been put to use as a useful means for preventing information leakage.
Meanwhile, techniques for preventing information leakage from printed matters printed on paper media and the like have not been developed sufficiently. In fact, it has been said that about half of information leakage is from printed matters, and there have been an urgent need for developing techniques for preventing information leakage from printed matters as well as for digital data.
Specific examples of needs for measures for preventing information leakage for printed matters include bills for purchased items, statements for credit cards and the like, medical records, school records, name lists, and the like, and prevention of information leakage by encrypting their important sections is required.
Against this background, a technique for encrypting or encoding printed matters and images has been developed as one of security measures for paper media. In this technique, pixel values of an area specified on a printed matter or an image are regularly converted, and encryption or encoding of the printed matter or the image is performed by generating a particular pattern corresponding to the conversion of the pixel values. At this time, on the four corners of the encrypted or encoded area, markers indicating the positions are placed. A reader that performs decrypting or decoding of the printed matter or the image for which encryption or encoding has been done as described above is able to perform decryption or decoding processing by detecting the markers placed on the area for which the encryption or encoding has been performed so that the coordinate of the area can be found and decryption or decoding processing can be performed.
In such a technique, when markers are placed on a printed matter or an image at the time of encryption or encoding, the original pixel values of the area on which the makers are placed may be overwritten by the markers. For this reason, at the time of decryption or decoding, the original pixel values of the area that were overwritten by the markers need to be recovered in one way or another. Otherwise, in the decrypted or decoded printed matter and image, the pixel values of the position would be missing, reducing the quality.
Here, a printed matter or an image for which encryption or encoding has been performed using the technique described above is supposed to be distributed not only through electronic media but also through paper media, and is subject to various quality deterioration. For this reason, recovery of the area that has been overwritten by the markers needs to be performed assuming quality deterioration of the printed matter and image. Furthermore, the recovery performance of the area that has been overwritten by the markers is also dependent on the image characteristics of the area that is overwritten (whether it is a background area or an area in which important information exists, etc).
However, conventionally, there has been no effective known recovery technique of a marker area in consideration of quality deterioration and image characteristics of a printed matter or an image.
Patent Document 1: Japanese Patent Application No. 2007-143301